This invention relates to single phase, alternating current induction motors and more particularly to single phase fractional horsepower motors which switch from a high torque starting mode to a high efficiency running mode.
Single phase, alternating current induction motors conventionally have a main field winding and a start field winding, the windings being angularly displaced on a stator core and being energized by phase displaced currents to provide the starting torque for the motor. This phase-displacement current requirement can be provided, for example, by connecting a resistor or a capacitor in series with the start field winding which is de-energized along with the start winding when the motor approaches operational speed.
In certain applications, however, such as in hermetically sealed compressor motors as can be used in refrigerators, a capacitor is put in series with the start field winding and both remain energized during the operation of the motor to provide high efficiency of operation. With this circuit configuration often the starting torque is insufficient so an impedance element such as a resistor or a capacitor with a switch is put in parallel with the earlier mentioned run capacitor with actuating means such as a potential relay to disconnect the impedance element after starting. It is to be understood that a current relay or even a centrifugal switch could be used as actuating means. The addition of the impedance element provides a starting and running circuit with a high torque starting mode which is de-energized when the motor reaches a predetermined condition. This circuit has a major drawback especially when the impedance element is a capacitor in that stalling of the motor or a momentary interruption in the line voltage occurring during or immediately following starting will cause the actuating means to momentarily close and open the switch contacts thereby applying a current surge from the capacitors through the switch contacts which may cause welding and switch failure.
A recent attempt to overcome this contact welding problem involves the use of a thermal relay as described in U.S. Pat. No. 3,725,757. This approach, however, has the disadvantage of providing a delay before restarting as well as being complex and costly.
It is an object of this invention to provide an improved starting and running circuit for a single phase motor. It is another object of this invention to provide a starting and running circuit for a single phase motor which provides high starting torque and efficient running operation while doing away with the problem of contact welding upon motor stalling or momentary interruption of the line voltage during or immediately following starting. Other objects and features of this invention will be in part apparent and in part pointed out hereinafter.
Briefly, the single phase motor starting and running circuit of this invention with a main field winding and a start field winding connected in parallel coupled across a source of single phase, alternating current potential comprises a single pole double throw switch in series with the start field winding to switch between a start impedance element and a run capacitor and actuation means for sensing motor speed or another desired characteristic to cause the switching from the start mode to the run mode. Upon a single phase, alternating current potential being applied across the main winding and start winding, the single pole double throw switch is normally positioned or is caused to be positioned by the actuation means to energize the start impedance element for good starting torque. When the motor reaches a predetermined condition the actuation means switches the start impedance element out of the circuit and energizes the run capacitor. If motor stalling should occur or a line interruption should take place during or immediately following starting, the actuation means will cause the single pole double throw switch to change back to the starting mode but this cannot cause contact welding problems as in previous designs because there is no current path in the circuit in which the start winding impedance is not in series with the contacts. This starting and running circuit never has the run capacitor in a continuous current loop with the start impedance element so no current surge between them is possible to cause contact welding. The start winding impedance is always in series with the contacts. Also, the motor can be restarted without delay.
Other objects and features of the invention will become more readily understood from the following detailed description and appended claims, when read in conjunction with the accompanying drawings in which like reference numerals designate like parts throughout the figures thereof.